1) Choleragen activates adenylate cyclase by ADP-ribosylating an arginine-like residue in the regulatory component of the cyclase. The toxin must be reduced by thiol to be active; its activity was enhanced by thiol: protein disulfide oxidoreductase. 2) GTP enhanced the activation of adenylate cyclase by choleragen by increasing ADP-ribosylation of the toxin substrate, enhancing stability of the ADP-ribosylated enzyme, and facilitating expression of the activated enzyme in the assay. 3) NAD:arginine ADP-ribosyltransferases were identified in human erythrocytes and rat liver and exhibited characteristics similar to the transferase from avian erythrocytes. The activity of the erythrocyte ADP-ribosyl-transferases was enhanced greater than 10-fold by micromolar histones. At these concentrations, histones activated the enzyme without serving as ADP-ribose acceptors. Certain inorganic salts, with chaotropic salts being the most active, increased ADP-ribosyltransferase activity to the same extent as that observed with histones. Activation by salt resulted in the conversion of inactive oligomeric forms of the transferase to active protomeric forms. The activated transferase in the presence of salt no longer responded to histones.